1. Field of the Invention
The present invention relates to a protective relay for a power transformer used in a power transmission system.
2. Description of the Prior Art
An example of conventional apparatus of this type is disclosed in a publication entitled "Protective Relay Engineering", Chapter 10, 10.1, FIG. 10.4, published by the Japanese Society of Electric Engineering. FIG. 1 shows the arrangement of such apparatus which includes a terminal 1 for receiving a differential input signal, a terminal 2 for receiving a restraint signal, a ratio-differential current detection device 3, a second harmonic component detector 4, and a logical AND gate 5 for taking the logical product of the outputs of the ratio-differential current detection device 3 and the second harmonic component detector 4, all in combination constituting a ratio-differential current detector 6.
FIG. 2 shows the waveform of the excitation rush current created when a transformer is connected to the power line, FIG. 3 shows the waveform of the excitation current created when an excessive voltage is applied to a three-phase transformer, FIG. 4 is a graph showing the excessive application voltage ratio V/V.sub.N of a transformer plotted against the allowable application time length, and FIG. 5 is a graph of the transformer characteristics showing the excessive application excessive voltage ratio V/V.sub.N plotted against the excitation current. The waveform drawn by the dashed line in FIGS. 2 and 3 indicates the fundamental component.
In operation, when a voltage is applied to the transformer, an excitation rush current with the waveform shown in FIG. 2 flows, causing the detector 4 to detect that the differential current includes an excessive amount of the second harmonic component with respect to the fundamental component and, in consequence, disable the AND gate 5 thereby to hold the operation of the relay. If an excessive voltage over the rated voltage is applied to the transformer, an excitation current with the waveform as shown in FIG. 3 flows in the transformer. The excitation current over a certain threshold magnitude operates on the ratio-differential current detection device 3 to enable the AND gate 5, so that the protective relay is activated. Namely, the transformer having the excessive application voltage ratio V/V.sub.N vs. application time length characteristics as shown in FIG. 4 withstands a transitional excessive voltage, but the relay operates instantaneously in response to such excitation current characteristics due to an excessive application voltage as shown in FIG. 5.
The conventional protective relay arranged as mentioned above is apt to respond erroneously to a voltage in excess of the rated voltage applied to the transformer, even though no internal fault current flows in the transformer.
In general, the excitation current of a transformer includes a great amount of the fifth harmonic component, taking on the waveform as shown in FIG. 3. A single-phase transformer is known to create a large amount of the third harmonic component and other odd harmonic components. Whereas, in a three-phase transformer with a phase difference of 120.degree. between each excitation current on the common time axis, the third, sixth and ninth harmonic components of the zero-phase component cancel with each other, resulting in a large proportion of the fifth harmonic component.